Peanut Butter Fudge Brownies

(18 1⁄2 oz [525 g]) box brownie mix + ingredients needed to prepare brownies

cups (375 g) chocolate peanut butter cups, such as Reese’s, halved (about 35)

cups (480 g) peanut butter morsels, found near the chocolate chips

[415 ml]) can sweetened condensed milk

vanilla extract